Carbohydrates are large molecules with the molecular formula C$_x$(H$_2$O)$_y$. In which of the following pairs is x not equal to y?

Analyze the question: Carbohydrates have the general formula Cx(H2O)y. We need the pair where x ≠ y in the formula, i.e. where the number of carbons does not equal the number of water units.

Formulas:

• Glucose: C6H12O6 = C6(H2O)6, so x = y
• Fructose: C6H12O6 = C6(H2O)6, so x = y
• Galactose: C6H12O6 = C6(H2O)6, so x = y
• Sucrose: C12H22O11 = C12(H2O)11, so x ≠ y
• Maltose: C12H22O11 = C12(H2O)11, so x ≠ y
• Starch: (C6H10O5)n = C6n(H2O)5n, so x ≠ y

Check each option:

• A. Glucose (x = y) and starch (x ≠ y): the pair is mixed, so not both unequal.
• B. Maltose (x ≠ y) and starch (x ≠ y): both have x ≠ y. ✔
• C. Sucrose (x ≠ y) and fructose (x = y): mixed.
• D. Galactose (x = y) and starch (x ≠ y): mixed.

Answer: Only option B has both members with x not equal to y.

Tip: Monosaccharides (C6H12O6) always have x = y. Disaccharides (C12H22O11) and polysaccharides (C6H10O5)n always have x ≠ y, because each condensation loses water. So the right pair is the one with no monosaccharide in it.